Joined connection on a two-piece steel piston and joining method

ABSTRACT

A piston, in particular, a gallery-cooled piston for an internal combustion engine has a piston bottom part and a piston upper part which are connected in a joining process to produce the piston. The piston upper part has at least one ring zone and an inner wall. The piston bottom part has a radially circumferential oblique surface at the end which faces the piston upper part. The oblique surface, interacting with the inner wall of the piston upper part, brings about a centering action during the joining process of the piston parts. A joining method for manufacturing a piston is also disclosed.

BACKGROUND

The disclosure relates to a piston for an internal combustion engine anda method for producing a piston for an internal combustion engine.

From DD 123 962, it is known that pistons for internal combustionengines consist of at least two parts. These two parts are firstly aring section that forms the upper land, amongst other things, andincludes at least one, or two or more ring grooves, and a base sectionthat, in an intrinsically known way, forms the piston pin bore, thepiston bosses, and the piston skirt. These two parts are joined in asuitable shape. The ring element and the base section are shaped in sucha manner that a cooling gallery is formed after the two parts arejoined. In addition, the piston may, but does not have to, include acombustion bowl.

DE 10 2010 056 220 A1 discloses a welded connection for producing apiston consisting of at least two individual parts. Assembling the partsof the piston in the correct location is extremely complex in thisinstance because a positioning device is lacking.

DE 10 2007 036 236 A1 discloses an assembled, specifically two-part,piston for an internal combustion engine having a piston upper part anda piston lower part that are joined by means of a thread. However,joining the piston parts by means of a threaded connection is extremelylabor-intensive and therefore extremely cost-intensive.

Known joining methods are, for example, press-fitting, soldering,welding, bolting, pinning, or similar. It is essential that asingle-piece piston is created from the two individual parts mentioned(ring section and base section) after the two parts are joined, wherethis single-piece piston gives rise to an operational piston after it iscompleted in the usual way with rings, pins, pin lock rings and similarparts.

In the case of the previously known joining methods, the disadvantage isalways present that the two components to be joined have to be alignedextremely precisely in order to be able to perform the joining method.

What is needed is to create a piston for an internal combustion engine,in particular a cooling-gallery piston that can be produced simply; butat the same time with great precision, without the need for costlyreworking after the two components have been joined.

SUMMARY

In accordance with the disclosure, provision is made for the twocomponents (ring element and base part, also called the lower part) tocenter themselves when they are brought together and to form a requisitegap for a joined connection (in particular, a material-to-materialjoined connection, for example, soldering or welding) as the result ofmachining.

The present disclosure relates to a piston, in particular acooling-gallery piston for an internal combustion engine, having apiston lower part and a piston upper part that are joined to form onepiston in a joining process, wherein the piston upper part has at leastone ring belt and an inner wall, wherein provision is made for thepiston lower part to have a radially circumferential oblique surface atthe end facing the piston upper part that interacts with the inner wallof the piston upper part to bring about a centering action during thejoining process.

In one aspect, the piston upper part can be designed such that itincludes the subsequent ring belt and the subsequent edge zone of thebowl. This piston upper part is designed as a ring with an invertedU-shaped profile having legs of different lengths. The longer, outer leglater forms the ring belt. The subsequent edge zone of the bowl isformed by the shorter, inner leg. The advantage of this is that asuitable material differing from the piston lower part can be used inhighly stressed areas.

It is further provided that the piston lower part can have at least onesliding surface that interacts with the inner wall of the piston upperpart during the joining process. This at least one sliding surfaceoffers the advantage that low force is to be applied during the joiningof the piston lower part and the piston upper part.

It is further provided that in the area of at least one joint seam atleast one seat can be provided to create a gap. As the result of the atleast one seat, a gap is created with defined dimensions.

It is further provided for the gap adjacent the seat to serve to receivebrazing material.

In addition, a repository can be provided to receive soldering material.The gap forms a reservoir for process media in conjunction with theseat. It is advantageous that this reservoir can be filled selectively,for example, before the joining process. After the joining, the processmedium, for example, soldering material, is available at the effectivelocation in the correct dosage. After carrying out a soldering process,for example, to join the piston parts, the excess soldering material canbe removed along with partial areas of the reservoir.

It is a characteristic of the piston that a circumferential collar isprovided on the piston upper part interacting with a seat as a limitduring the joining process. Advantageously, the precise, subsequent(height) dimension of the piston is defined by the collar. A “collision”of the piston parts beyond the limit formed by the collar and the seatis possible only if a definable force limit is exceeded. In the joiningprocess the specified force limit cannot be exceeded.

In one aspect, provision is made for the collar to be removed followingcompletion of the joining operation. Any traces of handling on thecollar from the joining process are eliminated by removing the collar.The precise dimension of the piston can also be adjusted by removing thecollar. The collar acts as an aid in finding the exact position of thepiston lower part and the piston upper part with respect to each otherin the joining process and it can be removed after joining.

Furthermore, provision can be made for the piston upper part to form atleast one ring belt after the joining procedure. The ring belt serves toreceive piston rings in a known manner.

The method for producing a piston, specifically, a cooling-gallerypiston for an internal combustion engine, comprises the following steps:

-   -   preparing the piston upper part and the piston lower part, and    -   the piston parts, wherein provision is made during the joining        process that the piston upper part and the piston lower part are        centered.

Centering increases process reliability when joining piston upper partand piston lower part to form one piston. As a result of centering, thepiston components “find” themselves in the correct position with respectto each other while they are being moved towards each other.

The piston upper part and piston lower part slide on each other into thecorrect final position to complete the joining process. Centeringensures that the midpoint toward the piston stroke axis of the pistonupper part lies above the midpoint toward the piston stroke axis of thepiston lower part.

In one aspect of the production method, there is provided that at leastone seat for the joining procedure is removed in at least one additionaloperational step. The seat serves to find a defined position of pistonlower part to piston upper part and can therefore be removed after thejoining process is completed.

For the sake of completeness it should be mentioned that the twocomponents to be joined may consist of the same material or of differentmaterials. In addition the two components can be produced by the samemethod or by different methods (e.g. forging, casting or similar).

DETAILED DESCRIPTION OF THE DRAWING

In what follows, aspects of the joined connection of a two-part pistonare explained in more detail using the appended Figures in which:

FIG. 1 shows a sectional view of a first aspect of the piston prior tojoining;

FIG. 2 shows a sectional view of the piston after being joined;

FIG. 3 shows a sectional view of a completed piston;

FIG. 4 shows a sectional view of a second aspect of the piston;

FIG. 5 shows a sectional view of the piston of FIG. 4 after beingjoined;

FIG. 6 shows a sectional view of the piston of FIG. 5 prior to thegroove being closed; and

FIG. 7 shows a sectional view of the finished piston shown in FIGS. 4-6.

DETAILED DESCRIPTION

FIGS. 1 to 3 show a first aspect of a piston 1, and FIGS. 4 to 7 show afurther second-aspect of a piston 3. For both aspects, identicalreference numerals are used for identical elements.

In the following description of the Figures, terms such as above, below,left, right, in front, behind, etc. refer solely to the selectedrepresentative example and position of the device and other elements inthe respective Figures. These terms are not to be understood in arestrictive sense, that is to say, these references can change as theresult of different positions and/or mirror-image layout or similar.

FIG. 1 shows a piston 1 consisting of a piston lower part 2 and a pistonupper part 3 designed as a ring prior to joining. The piston upper part3 has a seat 4. Additionally the piston upper part 3 may have arepository 5 that can serve to hold soldering paste, for example. Inaddition, the piston lower part 2 has an oblique surface 6 runningradially around the outer circumference. F identifies the direction forjoining the piston lower part 2 and the piston upper part 3. Whenjoining the piston lower part 2 and the piston upper part 3, a radiallycircumferential inner wall 7 of the piston upper part 3 slides pastradially circumferential sliding surfaces 8 of the piston lower part 2.In addition, the ring-shaped piston upper part 3 has a collar 14,oriented radially circumferential to the piston center. A radiallycircumferential stop 15 is located in the upper end area of the pistonlower part 2. The joining procedure is complete when the lower area ofthe inner wall makes contact with the oblique surface 6 and the collar14 makes contact with the stop 15. The oblique surface 6 assumes thefunction of a centering device during the joining procedure. Additionalmaterials, such as solder paste, may be, but do not have to be,introduced into the repository 5 prior to the joining procedure. It isalso conceivable to include materials or substances in the repository 5that serve for the later material-to-material connection between thepiston lower part 2 and the piston upper part 3.

FIG. 2 shows the later piston 1 after the joining procedure of thepiston lower part 2 and the piston upper part 3. It can be clearly seenthat the inner wall 7 of the ring-shaped piston upper part 3 has madecontact with the radially circumferential oblique surface 6 of thepiston lower part 2. Furthermore, the radially circumferential collar 14of the piston upper part 3 has made contact with the similarly radiallycircumferential stop 15 of the piston lower part 2. As the result ofjoining the piston lower part 2 to the piston upper part 3, a radiallycircumferential open space has formed that can act as a cooling gallery13. Furthermore, the piston lower part 2 has a bowl 11 in which thecombustion process takes place when the internal combustion engine isoperating.

FIG. 3 shows the now finished, one-piece piston 1. Joining seams 17 areclearly recognizable that designate the transition zone between theformer piston lower part 2 and the piston upper part 3. The piston 1 wasmachined in one area 18 such that a radially circumferential flatsurface was created to the greatest extent possible. To achieve this,the collar 14 and parts of the stop 15 were removed. In addition, aradially circumferential ring belt 16 was introduced in the upper areaof the piston 1. A material-to-material joint can be made in the areabetween the oblique surface 6 of the piston lower part 2 and the pistonupper part 3, for example, by welding or soldering. A tight join betweenthe piston upper part 3 and the piston lower part 2 is advantageous inorder to prevent the escape of cooling medium from the cooling gallery13.

The second aspect of a piston 31 is described starting with FIG. 4. Apiston lower part 32 again has a radially circumferential obliquesurface 6. In profile, the ring-shaped piston upper part 33 has theshape of an inverted “U” with legs 19, 20 of unequal length. The longer,outer leg 19 is used to form the ring belt 16 after the joiningprocedure. The shorter leg 20 is used to form a bowl edge ring 12 afterthe joining procedure. The piston lower part 32 has a radiallycircumferential web 21 in the area of a bowl 11 formed after the joiningprocess. In addition, a sliding surface 8 and an oblique surface 6 areformed at the upper outer circumference of the piston lower part 32.When the piston upper part 33 and the piston lower part 32 are joined inthe direction F, an inner wall 7 of the piston upper part 33 slidesalong the sliding surface 8 of the piston lower part 32. The joiningprocess ends when the lower end of the inner wall 7 of the piston upperpart 33 comes into contact with the oblique surface 6 of the pistonlower part 32. The joining process additionally comes to a stop when thelower end of the short leg 20 of the piston upper part 33 comes intocontact with the web 21 of the piston lower part 33.

FIG. 5 shows the piston 31 after the joining process. An optionalcontact surface 9 can be provided between the web 21 and the lower endof the leg 20 to form a soldering gap. Centering 10 of the piston upperpart 33 and the piston lower part 32, as already described with thefirst aspect, is carried out through the interaction of the obliquesurface 6 with the lower end of the inner wall 7. In the area of thesoldering gap, formed by the contact surface 9 and in the area ofcentering 10, a positive-locking, friction-locking ormaterial-to-material joint between the piston lower part 32 and thepiston upper part 33 can be made, for example, a soldered or weldedjoint can be provided.

Following additional machining steps, the completed piston 31 is shownin FIGS. 6 and 7. It can be seen that centering has been maintainedafter the machining. During the machining of the edge zone of the bowl11, the contact surface 9 for the solder gap, where it existed, wasremoved. The joint seams 17 between the piston lower part 32 and thepiston upper part 33 are visible. A ring belt 16 was likewise created.The bowl 11 has been given its functional profile.

The joining method for a piston lower part and a piston upper part isnot restricted to the aspects described here.

What is claimed:
 1. A piston for an internal combustion engine, having apiston lower part and a piston upper part that are connected in ajoining process to form the piston, wherein the piston upper part havingat least one ring belt and an inner wall, comprising the piston lowerpart having a radially circumferential oblique surface at an end facingthe piston upper part that interacts with an inner wall of the pistonupper part to bring about a centering effect during the joining processof the piston upper and lower parts.
 2. The piston from claim 1, whereinthe piston lower part has at least one sliding surface that interactswith the inner wall of the piston upper part in the joining process. 3.The piston from claim 1, wherein at least contact surface is provided inthe area of at least one joining seam to form a gap.
 4. The piston fromclaim 3, wherein the gap adjacent the contact surface acts to holdsoldering material.
 5. The piston from claim 1, wherein a repository isprovided to receive soldering material.
 6. The piston from claim 1,wherein a circumferential collar is provided on the piston upper part asa limit during the joining process interacting with a stop.
 7. Thepiston from claim 6, wherein the collar is removed after completion ofthe joining procedure.
 8. The piston from claim 1, wherein the pistonupper part forms at least one ring belt after the joining procedure. 9.A method for producing a piston, for an internal combustion engine,having a piston lower part and a piston upper part that are connected ina joining process to form the piston, wherein the piston upper part hasat least one ring belt and an inner wall, the method comprising:preparing the piston upper part and the piston lower part; and joiningthe piston parts, characterized in that wherein, during the joiningprocess, centering of the piston upper part to the piston lower parttakes place.
 10. The method from claim 9, comprising: removing at leastone stop for the joining procedure in at least one additional proceduralstep.